1. Field of the Invention
The present invention relates generally to a welding method and filler material for joining stainless steel components and, more particularly, relates to a welding method and filler material capable of forming a clean weld metal with high corrosion resistance in petrochemical environments combined with high mechanical properties, such as tensile strength and toughness at low temperatures.
2. Background Art
Stainless steel is usually welded by using filler metals in the form of welding rod or wire with a chemical composition substantially the same as that of the base stainless steel. In doing so, some problems regarding depression of mechanical properties and corrosion resistance of the weld metal may arise due to the type of base metal being joined. This can be particularly troublesome when the welded assembly is used in corrosive environments, such as those found in the petrochemical industry, where hydrogen sulfide exposure is a problem. Such exposure can lead to excessive corrosion, especially on the weld metal. Excessive corrosion may eventually result in the premature failure of the weld.
For welded assemblies that are subject to high shear stress, such as impellers used in compressors, depression of the mechanical properties and excessive corrosion of the weld metal may not only result in weld failure but could also damage other parts of the compressor. The impellers must also maintain their properties at extreme temperature conditions, such as, for example, the extreme cold typically encountered during the winter months in the oil fields of Alaska or the North Sea.
Weld filler metal alloys for welding stainless steel are typically made of iron and various other alloying elements and typically include:
(1) Carbon, an excessive amount of which precipitates as carbides and consumes chromium and other functional alloying elements and lowers their effective content.
(2) Silicon, which, although a useful element as a deoxidizing agent, is detrimental to the toughness of the weld metal. In particular, more than 1% by weight silicon content accelerates the production of intermetallic compounds, for example, sigma phase, and lowers the corrosion resistance and toughness of the weld metal.
(3) Manganese is an effective deoxidizing element and increases solubility of nitrogen in the weld metal. More than 1.5% by weight manganese unfavorably lowers the corrosion resistance and toughness of the weld metal.
(4) Phosphorus is unavoidably brought from steel-making materials and remains in the resulting steel alloy as an impurity thereof. It is preferable to keep the phosphorus content as low as possible. More than 0.04% by weight phosphorus content lowers the pitting corrosion resistance and toughness and increases the sensitivity to high-temperature cracking.
(5) Sulfur, similar to phosphorus, is unavoidably introduced from steel-making materials and remains in the resulting steel alloy as an impurity. More than 0.02% by weight sulfur lowers the pitting corrosion resistance and increases the sensitivity to high-temperature cracking.
(6) Nickel is an austenite-stabilizing element and suppresses the formation of ferrite phase in the weld metal.
(7) Chromium is necessary in order to ensure sufficient corrosion resistance of the stainless steel. An excess of chromium promotes a precipitation of intermetallic compounds, for example, sigma phase, and greatly lowers the toughness of the steel.
(8) Molybdenum is an element that increases the pitting corrosion resistance of the weld metal. It can promote a precipitation of sigma phase. A suitable amount of molybdenum increases the pitting corrosion resistance of the weld metal, without promoting the precipitation of sigma phase.
(9) Copper improves corrosion resistance, in particular, resistance to sulfuric acid. However, copper lowers the toughness of the weld metal.
U.S. Pat. No. 6,042,782 to Murata et al. discloses a welding material for use in consumable or non-consumable electrode welding of stainless steel, capable of forming a weld metal with excellent corrosion resistance and mechanical properties. The welding material is a composite welding wire composed of a steel shell and a filler material enveloped by the steel shell.
U.S. Pat. No. 5,556,561 to Ishikawa et al. discloses a method of forming a weld joint between an austenitic stainless steel and a ferritic steel. This method forms a clean weld metal with high corrosion resistance and high mechanical properties, such as tensile strength and toughness.
Tough, corrosion-resistant, martensitic, stainless steel alloys, such as 13Cr-4Ni stainless steel, are preferred for use as impellers, such as those used in compressors intended for low temperature, petrochemical service. However, commercially available, prior art weld filler metals do not provide the requisite corrosion resistance in hostile environments and strength at extremely cold temperatures.
Accordingly, a need exists for the development of a method and filler material to weld tough, corrosion-resistant assemblies of stainless steel elements such as 13Cr-4Ni alloy material for uses, such as for impellers used in compressors, that maintain their properties in corrosive or extremely cold environments.
The present invention is directed to a metal alloy that can be used as a filler metal for the welding of stainless steel components into a final assembly, particularly type 13Cr-4Ni stainless steel components. The filler metal alloy for welding includes in % by weight: up to 0.02% carbon; up to 0.8% manganese; up to 0.02% phosphorus; up to 0.015% sulfur; up to 0.6% silicon; from 4.5%-5.5% nickel; from 0.4%-0.7% molybdenum; from 10%-12.5% chromium; up to 0.1% copper; and the balance being essentially iron and incidental impurities.
The metal filler alloy more preferably consists essentially of in % by weight: 0.0001-0.02% carbon; 0.0001-0.8% manganese; 0.0001-0.02% phosphorus; 0.0001-0.015% sulfur; 0.0001-0.06 silicon; 4.5-5.5% nickel; 0.4-0.7% molybdenum; 10-12.% chromium; 0.0001-0.01% copper; balance essentially iron and incidental impurities.
The present invention is further directed to a method of welding stainless steel components, which includes the steps of:
(a) providing the stainless steel components to be welded, preferably formed from a type 13Cr-4Ni stainless steel alloy;
(b) austenitizing the stainless steel components to be welded at a temperature between 1800xc2x0 F.-2000xc2x0 F.;
(c) welding the austenitized components, using conventional arc welding techniques, utilizing a filler metal which includes in % by weight: up to 0.02%, and more preferably 0.0001-0.02% carbon; up to 0.8%, and more preferably 0.0001-0.8% manganese; up to 0.02%, and more preferably 0.0001-0.02% phosphorus; up to 0.015%, and more preferably 0.0001-0.015% sulfur; up to 0.6%, and more preferably 0.0001-0.6% silicon; 4.5-5.5% nickel; 0.4-0.7% molybdenum; 10-12.5% chromium; up to 0.1%, and more preferably 0.0001-0.01% copper; and the balance being essentially iron and incidental impurities to provide a welded assembly; and
(d) tempering the welded assembly at a temperature between 930xc2x0 F.-1300xc2x0 F.
According to an embodiment of the method of the present invention, a compressor impeller is manufactured wherein the impeller components are stainless steel, preferably a type 13Cr-4Ni stainless steel, welded with the above-described filler metal alloy. The present invention is also directed to a compressor impeller made from components of stainless steel welded together by the filler metal of the present invention which is useful in low-temperature, petrochemical service providing improved mechanical and corrosion-resistant properties.
These and other advantages of the present invention will be clarified in the description of the preferred embodiment taken together with the attached drawings in which like reference numerals represent like elements throughout.